1. Field of the Invention
The present invention relates to an optical modulating device employing a material having an anisotropy of refractive index, and particularly, to an optical modulating device employing ferroelectric liquid crystals (referred to as "FLC" hereinafter) showing the anisotropy of refractive index. Such an optical modulating device is preferably used in a display apparatus which displays letters, characters or images.
2. Related Background Art
One mode of an optical modulating device (SSFLC) in which a ferroelectric liquid crystal layer is formed in a narrow gap (e.g. 1 to 2 .mu.m) between a pair of parallel boards and in which the surface effect of the boards provides bistability for the FLC (described in Appl. Phys. Lett. Vol. 36, No. 11 (1980) pp. 889-901) was expected to provide a wide range of application because of its quick response and memory effect.
The bistable FLC device shows two stable states in directions shifted a certain angle both ways from the axis direction (rubbing direction or the like) of the aligning surface, which is formed by a rubbing method or the like on the liquid-crystal-side surfaces of boards positioned on both sides of the liquid crystal layer. The tilt angle is called a cone angle (hereinafter referred to by .theta.c). When voltage is applied perpendicularly to the liquid crystal layer of the FLC device, the FLCs shift from one stable state to the other. This shift corresponds to the 2.theta.c rotation on a plane of the FLC layer of the principal axis of index ellipsoid of a material having anisotropy of refraction index. Although, strictly speaking, the main axis of the index ellipsoid does not always coincide with an axis of the liquid crystal molecules, it is supposed that the two axes are in the same direction, to simplify the description. Therefore, when polarized light comes into a bistable FLC device having a thickness corresponding to the operation of a halfwave plate, the rotating effect of polarization to the incoming polarized light in one of the two stable states is 4.theta.c different from that in the other. When such a bistable FLC device is placed between polarizers (such as polarizing plates) positioned in crossed nicols or parallel nicols, an ON/OFF ratio of transmitted-light quantity, i.e., transmittance ratio, contrast, between the two stable states reaches a maximum if EQU 4.theta.c=90.degree. (.theta.c=22.5.degree.).
FIG. 4 illustrates an FLC device having polarizing elements positioned in crossed nicols. In the figure, incident light Ein goes through a polarizer 1 to become polarized light E1, which goes into an FLC layer 2 having an effect equivalent to that of a halfwave plate. The direction of the polarization of the incident light E1 is the same as that of the axis 45 of the FLC molecules in one of the two stable states. An analyzer 3 is positioned perpendicular to the polarizer 1. Another FLC molecule axis 46 is that of the other stable state. The molecule axes 45 and 46 have cone angles of +.theta.c and -.theta.c, respectively, to an aligning axis 44, for example, determined by the rubbing method. When the FLC molecules are in one stable state 45, the polarized light is not rotated by the FLC layer 2 and is thus entirely blocked by analyzer 3. Black is thus expressed. When the FLC molecules are in the other stable state 46, the polarized light is rotated 4.theta.c by the FLC layer 2, and then it is transmitted by the analyzer 3 at a ratio of EQU sin.sup.2 (4.theta.c).
White is thus expressed.
Cone angles .theta.c in bistable FLC devices are substantially dependent on temperature. Thus, even if a device is positioned as shown in FIG. 4 at one temperature, the FLC molecule axis 45 in one of the two stable states tilts from the polarization direction of incident light at another temperature because of a change in cone angle. In such a case, the polarized light is rotated by the polarization effect of the FLC layer. Therefore, the contrast degrades because black is not fully expressed in the case where the polarizer 1 and the analyzer 3 are positioned in crossed nicols, or because white is not fully expressed in the case where they are positioned in parallel nicols.